1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator mounted in communications equipment such as a cellular phone, and the like.
2. Description of the Related Art
A temperature-compensated crystal oscillator mounted in communications equipment such as a cellular phone and the like has an AT-cut crystal resonator (crystal unit) in a frequency band of 10 MHz as the oscillation source thereof, and the AT-cut crystal resonator is provided with some type of frequency adjustment circuit, making up a temperature compensation circuit. The temperature characteristic of the AT-cut crystal resonator, as expressed by a cubic curve, is canceled out by the temperature compensation circuit, thus stabilizing oscillation frequencies.
Such a temperature-compensated crystal oscillator is broadly broken down into two types depending on the configuration of the temperature compensation circuit: an analog temperature-compensated crystal oscillator, and a digital temperature-compensated crystal oscillator.
The digital temperature-compensated crystal oscillator is made up of a one-chip semiconductor integrated circuit (IC) with a nonvolatile memory mounted thereon, featuring the capability of achieving temperature compensation in a wide temperature range, and the generation of frequencies with high precision.
However, the same has failed to come into widespread use owing to its drawback of high phase noise.
On the other hand, the analog temperature-compensated crystal oscillator is made up of an AT-cut crystal resonator having the characteristic of a substantially constant oscillation frequency in the temperature range of from 15 to 45.degree. C., and a series-parallel circuit formed by discrete components such as a capacitor and thermistor.
With the analog temperature-compensated crystal oscillator, temperature compensation is effected mainly in a lower temperature range not higher than 15.degree. C., and in a higher temperature range not lower than 45.degree. C., respectively, through combination of the temperature characteristics of components thereof, and most of the products in widespread use today are of this type.
Temperature compensation by use of an analog temperature-compensated crystal oscillator made up of a one-chip semiconductor IC (referred to hereinafter as one-chip analog temperature-compensated crystal oscillator) instead of through the combination of the temperature characteristics of individual components has very recently been reported in, for example, literature by Kuichi Kubo, et al., 1996 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM, p. 728-734.
According to a temperature compensation method using the one-chip analog temperature-compensated crystal oscillator, after conducting a detailed study on the temperature characteristic of an AT-cut crystal resonator using a thermostatic bath (thermostat), a constant of a cubic curve generation circuit for canceling out the characteristic is written to a nonvolatile memory.
All temperature-compensated crystal oscillators have recently been forced to face up to the challenge of reduction in size and cost. In addition to that, in the light of an increasing tendency for the adoption of a system called CDMA aiming at international sharing of a common communications system, demand for expansion in the range of temperature compensation has been on the increase.
The range of temperature compensation up to now has been from minus 30.degree. C. to plus 75.degree. C. even in the case of a specification requiring the widest range. However, the CDMA requires that the range be expanded from minus 30.degree. C. to plus 85.degree. C.
A problem with the analog temperature-compensated crystal oscillator in achieving temperature compensation by taking advantage of a combination of the temperature characteristics of the components thereof is unavailability of suitable components meeting demand for low cost yet capable of effecting temperature compensation on the higher temperature side, not lower than 75.degree. C. This makes it difficult to expand the range for temperature compensation with the use of the analog temperature-compensated crystal oscillator.
The digital temperature-compensated crystal oscillator has no problem in respect of the range of temperature compensation.
However, it has a problem in that it is difficult to lower the phase noise level as low as that for the analog temperature-compensated crystal oscillator.
The one-chip analog temperature-compensated crystal oscillator has, in theory, a high potential of meeting all such requirements. However, with the conventional one-chip analog temperature-compensated crystal oscillator, it is not easy to reduce the cost of writing data for temperature compensation, leading to a problem of difficulty in achieving reduction in the total cost thereof.
In all, with the configuration of the conventional temperature-compensated crystal oscillators, it is extremely difficult to satisfy all the requirements, and the present state is far from the one desired by telephone manufacturers.